The quantitative measurements of a plurality of physical variables like pressure, electric or magnetic field, or concentrations of chemical analytes are usually affected by temperature variations. In order to provide temperature corrections for the readings obtained in the measurements of said physical variables, the prior art uses a temperature sensor in addition to the sensor for the particular variable being measured.
The co-pending application Ser. No. 10/251,416 describes a system for measuring oxygen pressure and temperature essentially simultaneously using a single oxygen-sensing photoluminescent probe, for example a ruthenium complex or a platinumn porphyrin, and an improvement on a previously patented system referred to as the Thermally Activated Direct Absorption (TADA) system, whereby the probe provides an indication of temperature without interference from and independent of the oxygen pressure acting on it. The reason there is no interference can be understood by noting that the measurement of the oxygen pressure is based on the oxygen quenching of the photoluminescence of the probe material (that is, a decrease of the luminescence quantum efficiency), a processes which occurs after the absorption of the luminescence excitation light, whereas the physical process indicative of the probe temperature is a temperature-dependent light absorption process which occurs prior to the photoluminescence and is not, therefore, affected by any processes which affect the photoluminescence efficiency, provided that the photoluminescence intensity is measurable to the needed extent. This is an easily met requirement given the great sensitivity of light detectors for visible and near infrared radiation. Once the temperature has been determined, a known temperature correction factor can be applied to the oxygen pressure reading provided by the same probe. The instrument used for measuring temperature and oxygen pressure is therefore substantially simplified compared to the prior art using two probes.
The situation is fundamentally more complex when one wishes to measure temperature with the same photoluminescent probe used for measuring a physical variable which, unlike oxygen pressure, modifies the optical absorption and/or luminescence bands of the probe material. Such physical variables include strong electric and magnetic fields. The effects of these fields are manifested by spectral shifts of the position and/or polarization of relatively narrow electronic absorption bands in rare earth-doped crystals, glasses or semiconductors. Magnetic fields affect the polarized absorption properties of rare earth ions like trivalent terbium, a phenomenon referred to as the optical Faraday effect, or the position of the peak absorption wavelength of one or more narrow absorption and/or photoluminescence bands (Zeeman effect). The peak wavelengths of said band can also be shifted by strong electric fields (Stark effect). Other electric field effects, for example the Franz-Keldysh effect, cause a spectral shift of the absorption band of semiconductors. But such shift is also temperature-dependent, and this cross sensitivity has made unreliable the optical measurement of electric fields using the Franz-Keldysh effect.
The co-pending application Ser. No. 10/251,416 disclosed, inter alia, a method for measuring temperature which is not affected by the oxygen quenching of the photoluminescence of the probe material. The method involves the use two different luminescence excitation wavelengths for exciting the probe material to the same electronic level, and the measurement of the relative intensities of the luminescence generated by the two wavelengths. A fraction α1 of the excitation light of wavelength λ1 and power P1 is absorbed by the probe to generate a luminescence light emitted with an intensity I1, and a fraction αT of the excitation light of wavelength λ, and power PT is absorbed by the probe to generate a luminescence light emitted with an intensity IT. The two excitation wavelengths are chosen so that αT and, therefore, IT, have a much stronger temperature dependence than α1 and I1. The temperature is then a unique function of the ratio (ITP1/I1PT).
It turns out, as shown below, that ratio (ITP1/I1PT) is also a reliable temperature indicator even if the photoluminescent probe is under a physical field that generates a measurable broadening or shift of the position of the peak absorption wavelength of one or more absorption and/or photoluminescence bands of the probe material. Therefore, the instant invention makes it practical to use a single photoluminescent probe to measure not only temperature and oxygen pressure, but also temperature and other physical variables, including some variables which affect the spectral properties of the probe material, like electric or magnetic fields.